This Project is an open-label, randomized, three-treatment, three-period, single-dose crossover, relative bioavailability study in 6 patients receiving chronic oral morphine therapy. The purpose of this study is primarily to investigate the relative bioavailability of two formulations of Mylan fentanyl 100ug/h transdermal systems manufactured for Mylan by the University of Michigan School of Pharmacy under the direction of Bertek Inc, to Duragesic-100, 100ug/h manufactured by Janssen Pharmaceutical. Fentanyl transdermal systems will be worn for 3 days (72 hours). Blood samples will be collected before dosing and for 120 hours after patch application. The relative bioavailability of fentanyl transdermal systems to Duragesic will be assessed by a comparison of various pharmacokinetic parameters derived from the plasma concentration-time curves. In addition, statistical analysis will be performed to assess drug pharmacokinetic parameters.